1. Field of the Invention
The present invention relates to an image sensor, and particularly to an MOS image sensor that extends the dynamic range with respect to the amount of incident light.
2. Description of the Related Art
In contrast to a CCD image sensor that requires a dedicated process, MOS image sensors, such as the sensor of the present invention, have received considerable attention in recent years because they can be fabricated by standard MOS processes and therefore enable the advantages of low power consumption by means of a low-voltage and single-power supply and because they allow incorporation of peripheral logic and macros on a single chip.
FIG. 1 shows an example of a prior-art method of extending the dynamic range with respect to the amound of light by O. Yadid-Pecht and E. Fossum as reported in xe2x80x9cWide Intrascene Dynamic Range CMOS APS Using Dual Samplingxe2x80x9d, (IEEE Transactions on Electron Devicesxe2x80x9d, Vol. 44, No. 10, pp. 1721-1723 (October, 1997).
According to this prior-art example for extending the dynamic range with respect to amount of light, the signal charge of pixel 21 for row n and row (nxe2x88x92xcex94), which have different exposure times, is read out separately to each of first horizontal transfer register 22 above and second horizontal transfer register 23 below, and these are integrated off-chip.
The above-described method, however, results in an increase in circuit scale because it necessitates both upper and lower horizontal scan circuits. There is the additional drawback that system scale increases because the integration of two screens having different exposure times is realized by off-chip processing.
It is an object of the present invention to provide an image sensor that can realize an image having wider dynamic range with respect to the amount of light in which overexposure and underexposure are mitigated without an accompanying increase in circuit scale.
The first image sensor of the present invention is an image sensor that includes a semiconductor device having a semiconductor region and a diffusion layer formed within the semiconductor region having the opposite conductivity of the semiconductor region; that, after discharging carrier in the diffusion layer of the semiconductor device, causes light to be irradiated into the diffusion layer to generate carrier in the diffusion layer, outputs a signal to an output section based on the surface potential of the generated carrier, and measures the amount of incident light; and that includes:
a timing generation means for creating: a first exposure period for, when irradiating light into the diffusion layer and generating carrier inside the diffusion layer, irradiating the light into the diffusion layer and generating a first carrier inside the diffusion layer; a storage period after the first exposure period for moving the first carrier to a storage section; a second exposure period after the storage period for irradiating the light into the diffusion layer and generating a second carrier inside the diffusion layer; and a readout period after the second exposure period; and
a carrier integration means for, when outputting to an output section a signal based on the surface potential of carrier that is generated by the timing generation means and measuring the amount of incidence of light, integrating the first carrier and the second carrier in the readout period and reading out the integrated carrier.
Furthermore, in a first mode of application of the first image sensor; the operation of irradiating light into the diffusion layer during the first exposure period and generating the first carrier in the diffusion layer is carried out in a state in which the diffusion layer and storage section conduct, and the operation of moving the first carrier to the storage section during the storage period that follows the first exposure period is carried out in a state in which the diffusion layer and storage section are cut off. In addition, carrier in the first image sensor that is contained in the diffusion layer and storage section is discharged before the second exposure period by means of a reset transistor that is connected to the power supply.
In a second mode of application of the first image sensor of the present invention, the operation of irradiating light into the diffusion layer during the first exposure period and generating the first carrier in the diffusion layer is carried out in a state in which the diffusion layer and storage section are cut off, and the operation of moving the first carrier to the storage section during the storage period that follows the first exposure period is carried out in a state in which the diffusion layer and storage section conduct.
The second image sensor of the present invention is an image sensor that includes a semiconductor device having a semiconductor region and a diffusion layer formed inside the semiconductor region having the opposite conductivity of the semiconductor region; that, after discharging carrier in the diffusion layer of the semiconductor device, causes light to be irradiated into the diffusion layer to generate carrier in the diffusion layer, outputs a signal to an output section based on the surface potential of the generated carrier, and measures the amount of incident light; and that includes:
a timing generation means for creating: a first exposure period for, when irradiating light into the diffusion layer and generating a first carrier, irradiating the light into the diffusion layer and generating carrier in the diffusion layer; a storage period after the first exposure period for moving a portion of the first carrier to a storage section and leaving first carrier in the diffusion layer; a second exposure period after the storage period for irradiating light into the diffusion layer and generating the second carrier inside the diffusion layer, and a readout period after the second exposure period; and
a carrier integration means for, when outputting to an output section a signal based on the surface potential of the generated carrier and measuring the amount of incidence of light, reading out carrier that is the sum of the second carrier and the first carrier that is left in the diffusion layer during a readout period.
In the above-described first and second image sensors of the present invention, a modification is possible in which carrier that is contained in the diffusion layer and storage section is discharged before the first exposure period by means of a reset transistor that is connected to the power supply; the period that extends-from the first exposure period to the second exposure period is positioned within the preceding readout period; and the first exposure period is longer than the second exposure period.
The third image sensor, which expands on the first image sensor of the present invention, is an image sensor that includes: a semiconductor device having a semiconductor region and a diffusion layer formed inside the semiconductor region having the opposite conductivity, of the semiconductor region; that, after discharging carrier in the diffusion layer of the semiconductor device, causes light to be irradiated into the diffusion layer to generate carrier in the diffusion layer, outputs a signal to an output section based on the surface potential of the generated carrier, and measures the amount of incidence of light; including:
a timing generation means for creating: when irradiating light into the diffusion layer and generating carrier inside the diffusion layer, a plurality of exposure periods that do not mutually overlap for irradiating light into the diffusion layer and generating carriers that correspond to the plurality of exposure periods inside the diffusion layer; a storage period for moving a preceding carrier that was generated inside the diffusion layer, in the one preceding exposure period of the plurality of exposure periods that relatively preceded to a storage section after the preceding exposure period; a succeeding exposure period after the storage period for irradiating the light into the diffusion layer after the preceding exposure period and generating a succeeding carrier inside the diffusion layer; and a readout period after the succeeding exposure period; and
a carrier integration means for, when outputting to an output section a signal based on the surface potential of the generated carrier and measuring the amount of incidence of the light, integrating, in the readout period following the last exposure period of the plurality of exposure periods, the carrier that was stored in the storage section up to the exposure period immediately preceding the last exposure period and the carrier that was generated inside the diffusion layer in the last exposure period.
Next, the fourth image sensor, which expands on the second image sensor of the present invention, is an image sensor that includes a semiconductor device having a semiconductor region and a diffusion layer formed inside the semiconductor region having the opposite conductivity of the semiconductor region; that, after discharging carrier in the diffusion layer of the semiconductor device, causes light to be irradiated into the diffusion layer and carrier to be generated inside the diffusion layer, that outputs a signal that is based on the surface potential of the generated carrier to an output section and measures the amount of incident light; and that includes:
a timing generation means for creating: when irradiating light into the diffusion layer and generating carrier in the diffusion layer, a plurality of exposure periods that do not mutually overlap for irradiating light into the diffusion layer and generating carriers in the diffusion layer that correspond to the plurality of exposure periods; a storage period, which follows the relatively preceding exposure period of the plurality of exposure periods, for moving to a storage section a portion of preceding carrier that was stored in the diffusion layer in exposure periods up to the preceding exposure period; a succeeding exposure period for simultaneously leaving preceding carrier in the diffusion layer and, after the storage period, irradiating light into the diffusion layer after the preceding exposure period and generating a succeeding carrier in the diffusion layer; and a readout period after the succeeding exposure period; and
a carrier integration means for, when outputting to an output section a signal based on the surface potential of generated carrier and measuring the amount of incidence of light, reading out carrier that is the sum of the preceding carrier that remained in the diffusion layer until the exposure period immediately preceding the last exposure period and the succeeding carrier that was generated in the diffusion layer in the last exposure period in the readout period following the last exposure period of the plurality of exposure periods.
In the above-described third and fourth image sensors of the present invention, a modification can be adopted in which the preceding exposure period of the plurality of exposure periods is a longer period than an exposure period that is positioned later; and in which a period that extends over a plurality of exposure periods is positioned within a preceding readout period.
A modification is adopted in common to each of the above-described first, second, third, and fourth image sensors of the present invention in which the diffusion layer constitutes the pixels of the image sensor and the storage section is provided inside pixels in correspondence to the diffusion layer.
The first, second, third, and fourth image sensors of the above-described present invention have the following circuit configuration:
The circuit configuration of the first, second, third, and fourth image sensors include unit pixels that are each composed of:
a photodiode of a structure that converts irradiated light to electrons, has its anode connected to ground, and extracts electrons from its cathode;
an amplification transistor having its gate connected to the cathode of the photodiode, its drain connected to a power supply line, and its source connected to the drain of a readout transistor;
a reset transistor having its source connected to the cathode of the photodiode, its gate connected to a reset line, and its drain connected to a power supply line,
a pixel interior capacitance selection transistor having its drain connected to the cathode of the photodiode, its gate connected to the pixel interior capacitance selection line, and its source connected to pixel interior capacitance;
pixel interior capacitance having one end grounded and the other end connected to the source of the pixel interior capacitance selection transistor; and
a readout transistor having its drain connected to the source of the amplification transistor, its gate connected to a horizontal selection line, and its source connected to a vertical readout line.
The pixel interior capacitance is composed of a MOS transistor and is of a construction in which the source and drain of the MOS transistor are short-circuited and connected to ground and the gate is connected to the source of the pixel interior capacitance selection transistor. The reset transistor and pixel capacitance selection transistor are both depletion-type MOS transistors, and in this case, the potential of the reset transistor when OFF is higher than the potential of the pixel capacitance selection transistor when OFF.
As described hereinabove, by performing two exposures of different exposure periods, mixing the signals that are generated in each exposure period in pixels (holding the signal charge that is generated in the first exposure period in capacitance that is provided in pixels and mixing the signal charge that is generated in the second exposure period with the first signal charge in the pixels), and reading out, the image sensors of the present invention can obtain an image of wider dynamic range with respect to the amount of light with mitigated overexposure and underexposure because overexposed portions in the first exposure period are compensated by information of the second exposure period and underexposed portions in the second exposure period are compensated by information of the first exposure period.
In addition, the invention has the advantage of enabling an extension of dynamic range without reducing the frame readout speed because the above-described two exposures can be performed during readout of one frame.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.